A patent document, Japanese Patent Laid-Open No. 2008-271040 (Patent document 1) listed below discloses a route switching scheme that switches one bus to the other, i.e., to an alternative, bus, when one bus has abnormality and becomes interrupted, under control of a communication device that controls communication of plural Electronic Control Units (ECU) that are respectively connected to both of two specific buses. Such a communication device enables the communication via the alternative bus by limiting a data amount of the communication via the alternative bus to the minimum necessary amount, for avoiding an exclusive use of the alternative bus by only one ECU and for preventing an overload/high-load of the alternative bus.
However, the communication device in the patent document 1 is effective only when all of the ECUs are connected to two buses, which may put some restrictions on the bus structure. Further, when the alternative bus is used, the data amount is restricted.